Question

What is elevator music?

Answer 1

Elevator music is the music that is typically played while riding in an elevator in places such as a hotel. Generally, it is a slower jazz piece.

Answer 2

"Elevator music" is the general, mildly pejorative term for music that is written or
performed with the intention of having the music set the atmosphere or the mood
of a place, and be otherwise ignored and not call attention to itself. 

It's 'background music', 'dinner music' etc.  It's music in the gaseous state.

Real music has a beginning, a structure, a middle, a development, a melody,
some variations, a resolution, and an end. 'Elevator music' really doesn't need
any of these things.  It's built and performed in such a way that its effects on a
listener completely DON'T depend on when he comes in, how long he stays,
whether he tunes in or tunes out, and when he leaves.